The present invention relates to an oil pressure control system for an automatic transmission for automobiles, and, more particularly, to an improvement of the shifting performance of the transmission in its downshifting.
In an automatic transmission for automobiles having a fluid torque converter, a gear mechanism and a plurality of friction engaging means, the engagement of the friction engaging means is automatically changed over in accordance with operating conditions of the vehicle so as to establish the most desirable speed shift stage for the moment-to-moment operating conditions of the vehicle. Such a changing over control of the friction engaging means is effected by an oil pressure control means which has a shift valve adapted to be shifted by the balance of the throttle pressure, which varies in accordance with the amount of the accelerator pedal depression (i.e., the throttle opening), and the governor pressure, which varies in accordance with the vehicle speed, and which is adapted to select various speed shift stages in accordance with the contrast between the throttle pressure and the governor pressure, i.e., the accelerator depression pedal versus the vehicle speed. The shift valve is adapted to be shifted between a first shift position where it connects a low speed friction engaging means to a source of line pressure while it drains a high speed friction engaging means and a second shift position where it connects the high speed friction engaging means to the source of line pressure while it drains the low speed friction engaging means. Furthermore, in order to provide a certain overlapping between the disengagment and engagement of the low and high speed friction engaging means in the changing over of the two friction engaging means, which is desirable in order to accomplish smooth shifting between the two speed shift stages, the oil pressure control system generally includes low and high speed accumulators connected individually to the passages for supplying the line pressure to the low and high speed friction engaging means.
FIG. 1 shows an example of diagrams showing the variation of the oil pressures in the high and low speed friction engaging means, the rotational speed of the engine, and the output torque of the transmission during the downshifting of the transmission from a high speed stage to a low speed stage effected by the changing over of engagement of high and low speed friction engaging means, in a conventional automatic transmission having high and low speed accumulators. In FIG. 1, the curve A in the oil pressure diagram shows the variation of the oil pressure in the high speed friction engaging means (high clutch), and the curve B shows the variation of the oil pressure in the low speed friction engaging means (low clutch). In this diagram, portion `a` of the curve A corresponds to the operational region of the high speed accumulator, whereas portion `b` of the curve B corresponds to the operational region of the low speed accumulator. At point `c` the high clutch becomes substantially disengaged, while at point `d` the low clutch becomes substantially engaged. When a proper timing such as shown in FIG. 1 is maintained between the disengagement of the high clutch and the engagement of the low clutch, the rotational speed of the engine gradually increases after the disengagement of the high clutch until it reaches the value which corresponds to the momentary vehicle speed at the low speed gear ratio, whereupon the low clutch is just engaged, whereby the downshifting is accomplished relatively smoothly without causing negative torque in the output shaft of the transmission as shown in FIG. 1. However, the output torque of the transmission still undergoes a relatively large variation and the smoothness of such downshifting is not yet perfectly satisfactory. If the timing between the disengagement and engagement of the two friction engaging means becomes improper such as shown in FIG. 2, wherein the engagement of the low clutch is too early relative to the disengagement of the high clutch, the output torque attains momentarily a great negative value and causes an uncomfortable shock. Similarly, although detailed graphs are not shown in the drawing, if on the other hand the engagement of the low clutch is delayed relative to the disengagement of the high clutch, the engine races.
The operating performance of a high speed accumulator, and particularly the operating pressure thereof, is determined so that the shift shock during upshifting is softened while ensuring definite transmission of the driving torque in the operation by the high speed stage. Therefore, in downshifting, during the region `a` of the curve A in FIG. 1, the oil pressure in the high clutch is maintained at such a high level that no slippage can occur in the high clutch. In this case, therefore, the timing relation between the points `c` and `d` is the sole condition which determines smoothness of downshifting. However, it is very difficult to prescribe exactly in advance the proper timing relation between these two time points for every actual oil pressure control means, due to manufacturing tolerances, and furthermore the timing relation varies in accordance with the vehicle speed and the output torque of the engine when downshifting. In view of this, it is almost impossible to provide the optimum timing relation between the disengagement of the high speed friction engaging means and the engagement of the low speed friction engaging means over a wide range of operational conditions of the vehicle.